1. Field of the Invention
The present invention relates to electrical balances in which an electrical signal generated so as to null the balance electromagnetically is provided to a digital read out or other display for the balance, and more particularly to electrical balances of this type having a tare capability in which the weight of a sample holder can be electrically determined and thereafter subtracted from the combined weight of the sample holder and included sample to provide a direct indication of sample weight.
2. History of the Prior Art
It is known in the art to provide electrical microbalances for weighing relatively small masses to a high level of precision. Such balances typically generate an electrical signal having a magnitude representing the mass being weighed. The signal is provided to an appropriate display such as a digital read out for providing a visual or other indication of the weight of the mass being weighed. In many cases the electrical signal is also applied to an electromechanical arrangement in a servoing arrangement which provides the signal with a magnitude just sufficient to enable the electromechanical device to compensate for the weight of the mass and thereby null the balance.
Electrical balances of this type are frequently provided with tare systems whereby the weight of a sample holder can be determined and stored for later subtraction from the total weight of the sample holder and an included sample to thereby provide a direct indication of the sample weight. Examples of such arrangements are provided by U.S. Pat. Nos. 3,684,875 to Smith et al, 3,777,828 to Dietemeyer, 3,812,923 to Rock, 3,851,720 to Williams, 3,853,267 to Cadwell et al, 3,860,077 to Utzinger et al, 3,861,479 to Pryor, 3,888,321 to Wiiki et al and 3,962,570 to Loshbough et al.
Electronic tare systems of the prior art such as those described in the above-noted patents have certain inherent limitations which detract from their usefulness and may even render them unsuitable for certain applications. One of the most important limitations in such arrangements is that the tare weight corresponding to the weight of the sample holder is limited by the range of the balance. Thus if the range of the balance is 100 Mg, it may be difficult or impossible to operate in the tare mode using sample holders weighing much in excess of 100 Mg. This can be a severe limitation when it is considered that the sample holder often weighs as much as several times more than the samples therein being weighed. The weight of the sample holder is typically determined digitally with the digital value being stored for later subtraction from a digital value representing the combined weight of the sample holder and an included sample. Still other limitations in prior art tare systems arise from the fact that such systems are typically unipolar in the sense that the value or signal representing tare weight has a fixed polarity and is always generated by changes of fixed polarity in the value or signal.
Accordingly, it would be advantageous to provide a tare system for an electrical balance in which tare weight is not limited by the range of the balance.
It would furthermore be advantageous to provide a tare system which has a bipolar capability.
It would still furthermore be advantageous to provide a tare system which determines the tare weight in a relatively rapid, efficient and accurate fashion and which remains ready to enter a tare mode operation substantially unaffected by signal drifts and other common circuit conditions of that type.